Often old springs lose tension and dont hold pressure as they should. 
Or maybe you have one of the aftermarket High pressure springs and have too much pressure!
 
 
Product AttributesPart :   Oil Pressure ReliefBrand:    VWQuality:    Genuine VW
Part numer: 00-111115421A
 
FOR VEHICLESProduct Vehicle YearsKarmann Ghia    1960 - 1974Splitscreen    1960 - 1967Baywindow    1968 - 1979Beetle    1960 - 1979Beetle Cabrio    1960 - 1979